A. Field of Invention
The present invention pertains generally to speed detection systems for moving vehicles, and more particularly to automatic speed detection systems that are capable of capturing an image of a moving vehicle.
B. Description of the Background
Various methods have been used in the past to detect the speed of moving vehicles. For example, radar systems have been developed that measure the Doppler frequency of a wave reflected from a moving vehicle that is mixed with the transmitted wave. These systems have been implemented in the X, K and Ka microwave bands.
More recently, lasers have been used to detect the speed of moving vehicles. Lasers operate in a somewhat different fashion than radar systems. Laser speed detectors measure the time delay between the transmission of a series of pulses and a reflection of those pulses from the moving vehicle. This time delay is an indication of the distance of the moving vehicle from the laser speed detector. By taking several such readings over a predetermined time period, the speed of the vehicle can be determined. Laser range finders and speed detection systems, such as that disclosed in U.S. Pat. No. 5,359,404 issued Oct. 25, 1994 entitled "Laser-Based Speed Measuring Device," U.S. patent application Ser. No. 08/375,945 filed Jan. 19, 1995, entitled "Laser Range Finder Having Selectable Target Acquisition Characteristics and Range Measuring Precision" (attorney docket number 35014.8306), U.S. patent application Ser. No. 08/375,941 filed Jan. 19, 1995, entitled "Self-Calibrating Precision Timing Circuit and Method for Laser Range Finder" (attorney docket number 35014.8307), and U.S. patent application Ser. No. 08/375,810 filed Jan. 19, 1995, entitled "Automatic Noise Threshold Determining Circuit and Method for a Laser Range Finder" (attorney docket number 35014.8308) are specifically incorporated herein by reference for all that they teach. Although laser speed detection devices can provide highly accurate measurements of the speed of a moving vehicle, alone they do not provide automated features that a user may find desirable.
For example, a system that is capable of recording an image of a moving vehicle together with the speed of the vehicle would facilitate the issuance of speeding tickets by providing evidence of the identity, speed, time and location of the vehicle. Such evidence could then be communicated directly to the registered owner together with a speeding citation. Various prior art systems have been capable of producing this type of composite image. For example, prior art radar based systems have been capable of printing out various data parameters on a picture that relate to time, date, speed limit and actual speed of the moving vehicle. This information, typically, has been placed on an image of the vehicle in an analog fashion.
TeleTraffic of Norway has developed a laser speed detection system called "Lastec" that records the time, date, speed limit and actual speed of a moving vehicle (data parameters) on an image of the vehicle in an analog fashion. The Lastec device uses a magnetic tape storage device for recording low light levels require lower F-stops which results in a shorter depth of field that causes the image to be difficult to focus. On the other hand, off-the-shelf electronic automatic gain control (AGC) systems typically do not provide an adjustment range that is sufficient to obtain images in bright sunlight as well as very low light conditions. Typical AGC systems generate a gain control signal that is supplied to the video amplifiers to adjust the gain of the video amplifiers to produce a constant video signal. Such systems do not, however, provide a sufficient range of adjustment for bright sunlight and low light level conditions. Additionally, such systems are incapable of controlling an exposure for flash illumination. Both mechanical iris exposure systems and standard built-in AGC electronic exposure systems for video cameras fail to provide a method of obtaining a proper exposure for a flash illumination of a target. Exposure systems that are provided with video cameras are not capable of setting exposures for flash illumination that are capable of capturing a single-image frame during a flash illumination.
Proper alignment of laser based speed detection systems has also been found to be difficult in prior art systems. In heavy traffic situations, the narrow laser beam allows the user to specifically pinpoint a particular target and obtain data parameters such as speed and distance of the moving vehicle with a high degree of precision. This type of precision was not available in radar systems because of the width of the radar beam. However, such precision requires a highly accurate alignment and targeting system to insure that data is being obtained from the proper target. It is also desirable to have a portable system that is capable and storing the image and data. The combined data and image comprise an information record. The Lastec system codes each of the information records with a bar code so that the information record can be located on the tape at a later date.
Although such prior art systems, such as the Lastec system, are capable of storing a large amount of data on a magnetic tape, the use of tape-based systems in a field unit is complex and awkward. Tape systems are bulky, require extra cabling, and awkward to use if the tape is to be removed and used on a base unit to print images. Additionally, tape systems only provide serial access, rather than at random access, which further slows the retrieval time for images. Also, since the data parameters such as the date, time, speed and speed limit are recorded in an analog fashion on the image, there is no way to search this data. It has also been found that the reliability of tape systems can be affected by changes in temperature. Moreover, tape systems that could be used in a portable system suffer from low data transfer rates necessitating the use of a high data transfer rate storage device, such as a hard disk, in combination with the tape system, thereby unnecessarily adding complexity and expense to the overall system.
Obtaining the proper exposure of the image of a moving vehicle has also been found to be difficult. Because of the high rate of speed of vehicles that are being detected, as well as changing conditions, the light level intensity of the moving vehicle can vary in a short period of time. Mechanical iris-type systems use a variable F-stop to maintain a proper exposure of the image. However, the response time of such mechanical systems can be too slow to obtain a properly exposed picture. Additionally, of maintaining precise alignment. Prior art systems have required elaborate alignment procedures during setup and elaborate targeting methods during data acquisition. Alignment can also be more complex when the field unit must be disposed within a weatherproof unit.
Use of weatherproof enclosures has also limited the ability of the user to externally focus the camera system. A change in the focal distance of the video camera may be required because of a change in the target acquisition distance or a change in the illumination frequency. Prior art devices have not provided a simple and easy method of adjusting the focus, especially when the imaging system is contained in a waterproof enclosure.